Magnetic random access memory with dual spin torque reference layers

ABSTRACT

A magnetic data storage cell, applicable to spin-torque random access memory (ST-RAM), is disclosed. A magnetic cell includes first and second fixed magnetic layers and a free magnetic layer positioned between the fixed magnetic layers. The magnetic cell also includes terminals configured for providing a spin-polarized current through the magnetic layers. The first fixed magnetic layer has a magnetization direction that is substantially parallel to the easy axis of the free magnetic layer, and the second fixed magnetic layer has a magnetization direction that is substantially orthogonal to the easy axis of the free magnetic layer. The dual fixed magnetic layers provide enhanced spin torque in writing to the free magnetic layer, thereby reducing the required current and reducing the feature size of magnetic data storage cells, and increasing the data storage density of magnetic spin torque data storage.

BACKGROUND

Magnetic random access memory (MRAM), or spin torque RAM (STRAM), is anon-volatile solid-state data storage technology that has long shownpromise, but has posed challenges in achieving competitive levels ofstorage density.

The discussion above is merely provided for general backgroundinformation and is not intended to be used as an aid in determining thescope of the claimed subject matter.

SUMMARY

A magnetic data storage cell, applicable to spin-torque random accessmemory (ST-RAM), is disclosed. A magnetic cell includes first and secondfixed magnetic layers and a free magnetic layer positioned between thefixed magnetic layers. The magnetic cell also includes terminalsconfigured for providing a spin-polarized current through the magneticlayers. The first fixed magnetic layer has a magnetization directionthat is substantially parallel to the easy axis of the free magneticlayer, and the second fixed magnetic layer has a magnetization directionthat is substantially orthogonal to the easy axis of the free magneticlayer. The dual fixed magnetic layers provide enhanced spin torque inwriting to the free magnetic layer, thereby reducing the requiredcurrent and reducing the feature size of magnetic data storage cells,and increasing the data storage density of magnetic spin torque datastorage.

The Summary and Abstract herein provide an illustrative introduction tocertain aspects of selected embodiments, and are understood not todefine any limitations or implications for how the scope of the claimedsubject matter might be interpreted.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a perspective view of a magnetic data storage cell, withexploded views of magnetic layers of the cell with illustratedmagnetization directions, in accordance with an illustrative example.

FIG. 2 depicts a side plan view of a magnetic data storage cell, withschematic depictions of magnetization directions and of spin torquesbeing applied in the cell, in accordance with an illustrative example.

FIG. 3 depicts a graphical representation of a superposition of multiplespin torques acting within a cell as a function of angles betweenmagnetization directions, in accordance with an illustrative example.

FIG. 4 depicts a comparative graphical representation ofmagnetoresistance as a function of current for different magnetic cells,in accordance with an illustrative example.

FIG. 5 depicts a perspective view of a magnetic data storage cell, withexploded views of magnetic layers of the cell with magnetizationdirections, in accordance with another illustrative example.

FIG. 6 depicts a perspective view of a magnetic data storage cell, withexploded views of magnetic layers of the cell with magnetizationdirections, in accordance with another illustrative example.

FIG. 7 depicts a data storage system comprising magnetic data storagecells, in accordance with another illustrative example.

FIG. 8 depicts a method associated with a magnetic data storage cell, inaccordance with an illustrative example.

DETAILED DESCRIPTION

FIG. 1 depicts a perspective view of a magnetic data storage cell 100,with exploded views of magnetic layers 121, 122, 123 of the cell withillustrated magnetization directions, in accordance with an illustrativeexample. Many magnetic data storage cells such as magnetic cell 100 maybe included together in a data storage device or other data storagesystem, and configured for storing data, in an illustrative embodiment.In this illustrative embodiment, dual fixed magnetic layers 121, 123,also referred to as reference layers, having complementary magnetizationdirections, provide enhanced spin torque in writing to the free magneticlayer 122, by switching the magnetization direction of the free magneticlayer 122, as is further described below.

The enhanced spin torque provided by the dual fixed magnetic layers 121,123 enables the free magnetic layer 122 to be switched using a lowercurrent, relative to a magnetic cell with just a single fixed magneticlayer. This lower current allows reducing the feature size of themagnetic data storage cell 100, relative to a magnetic cell with just asingle fixed magnetic layer, and thereby enabling increased data storagedensity in a magnetic spin torque data storage that incorporates suchmagnetic data storage cells. In particular, magnetic cells with a singlefixed reference layer may require a larger current than can be providedwith integrated field-effect transistors (FET's) typical of integratedcircuits, while in contrast, magnetic cells with dual fixed magneticlayers with complementary magnetization directions may function with alower current that is within the normal current capabilities of anintegrated circuit. These aspects are further described below.

The magnetic layers of cell 100 include a first fixed magnetic layer121, a second fixed magnetic layer 123, and a free magnetic layer 122positioned between the first and second fixed magnetic layers 121, 123.Magnetic layers 121 and 123 are fixed in that their magnetizations areeach kept in a respective fixed direction, while magnetic layer 122 isfree in that its magnetization is left free to align in either of twoopposing directions along its easy axis, as further explained below.“Magnetization” may be understood to indicate magnetization direction asapplicable. Magnetic cell 100 also includes terminals 111, 113configured for providing a spin-polarized current through the stack ofmagnetic layers 121, 122, 123. Terminals 111, 113 are connected to node115 which may connect to additional signal lines (not depicted in FIG.1). The exploded views of the magnetic layers show magnetizationdirection 141 of fixed magnetic layer 121, indicative of the fixedmagnetization direction of magnetic layer 121; magnetization direction143 of fixed magnetic layer 123, indicative of the fixed magnetizationdirection of magnetic layer 123; and magnetization direction 142 of freemagnetic layer 122, indicative of the easy axis of magnetic layer 122.

The magnetization of magnetic layer 121 has a perpendicular orientation,also sometimes referred to as out-of-plane or vertical orientation, withthe magnetization oriented generally perpendicular to the flat, extendedcircular surfaces of the layer. The magnetization of magnetic layer 123has a diametrical orientation, also sometimes referred to as in-planeorientation, and oriented generally parallel to the flat surfaces of thelayer. As those skilled in the art will recognize, these descriptions,including “generally orthogonal” and “generally parallel” are simplifiedand do not account for the complete description of the magnetizationwithin each of the layers or indicate precisely orthogonal or preciselyparallel, but are useful in identifying the general orientation of themagnetizations, as would be understood by a person of ordinary skill inthe art when considering the relative orientations of the magnetizationdirections.

The magnetization directions 141, 143 of the fixed magnetic layers areeach depicted as solid lines, each with an arrowhead on only one endthereof, indicating that the magnetization directions of these magneticlayers are fixed, while the magnetization direction 142 of free magneticlayer is depicted as dashed lines with arrowheads on both ends thereof,indicating that this magnetization direction is indicative of an easyaxis, and the magnetization of the layer may be aligned with eitherpolarity of the easy axis with substantially equal stability. Ideallythis will be equal stability, or may be within nominal manufacturingtolerances of equal stability, such that if the magnetization directionof the free magnetic layer is disrupted and allowed to proceed in theabsence of environmental influences, the magnetization direction wouldbe equally likely to return to either direction in line with the easyaxis.

The easy axis of the free magnetic layer 122 is set in the perpendicularorientation, in alignment with the magnetization direction 141 of fixedmagnetic layer 121, in the illustrative embodiment of FIG. 1. Therefore,the first fixed magnetic layer 121 has a magnetization direction 141that is substantially parallel to the easy axis of the free magneticlayer 122, and the second fixed magnetic layer 123 has a magnetizationdirection 143 that is substantially orthogonal to the easy axis of thefree magnetic layer 122, in the illustrative embodiment of FIG. 1. Themagnetization direction 142 of the free magnetic layer 122 at any timeis predisposed to stable alignment with the easy axis, either parallelor antiparallel to the fixed magnetization direction 141 of magneticlayer 121. The magnetization directions are “substantially” parallel and“substantially” orthogonal in a sense that is further discussed below,with reference to FIG. 2.

The easy axis may be set by various techniques which may illustrativelyinclude shape anisotropy or magnetocrystalline anisotropy in the freemagnetic layer. In the fixed magnetic layers 121 and 123, themagnetization direction of each of the layers may be kept in a fixedorientation by any of various techniques, such as having the fixedmagnetic layers each have a substantially greater magnetic volume thanthe free magnetic layer, or by having the fixed magnetic layersmagnetically pinned, for example.

The orientation of the magnetization may also be a function of thedimensions of the layer. For example, absent magnetic pinning, a layerhaving an easy axis (and magnetization) in a diametrical magneticorientation generally correlates with a relatively larger radius andrelatively smaller vertical thickness, while a layer having an easy axis(and magnetization) in a perpendicular magnetic orientation generallycorrelates with a relatively smaller radius and relatively largervertical thickness.

In particular, having the perpendicular easy axis for the free magneticlayer 122 may increase the amount of anisotropy energy density of thefree layer relative to other easy axis orientations, in this particularembodiment. This may enable magnetic cell 100 to have a relatively smallsize with relatively larger magnetic stability, compared with otherorientations. A significant constraint on how small magnetic cell 100can be, while still functioning reliably, is resistance to randomreversals of the magnetization direction of free magnetic layer 122 dueto random thermal fluctuations. The stability of the magnetizationdirection of the free magnetic layer against thermal disturbances can bemodeled as K_(μ)V/kT, where K_(μ) is magnetic anisotropy energy density,V is the volume of the magnetic layer, k is Boltzmann's constant, and Tis temperature. In one illustrative embodiment, a value for K_(μ)V/kT ofat least approximately 60 (with no units, because x is a dimensionlessratio) is used as a design standard for maintaining the magneticstability of the free magnetic layer 122.

The magnetic layers 121, 122, 123 may be composed at least in part of aferromagnetic material. Examples of ferromagnetic materials that may beused for the compositions of the magnetic layers include iron, cobalt,nickel, Permalloy, Heusler alloys, or any other ferromagnetic materials.Heusler alloys that may be used may, for example, be composed of acombination of two parts of copper, nickel, cobalt, or a combinationthereof; one part manganese, chromium, iron, or a combination thereof;and one part tin, aluminum, silicon, arsenic, antimony, bismuth, orboron, or a combination thereof. Those skilled in the relevant arts willrecognize additional selections of materials that may be well-suited fora given application.

The intermediate, non-magnetic interlayer 131 is positioned betweenfixed magnetic layer 121 and free magnetic layer 122, and intermediate,non-magnetic interlayer 132 is positioned between free magnetic layer122 and fixed magnetic layer 123, in the illustrative embodiment ofFIG. 1. Interlayers 131, 132 may be configured to contribute todifferent technologies for magnetic cell 100, such as quantum tunnelingmagnetoresistance (TMR) or giant magnetoresistance (GMR), for example.One type of interlayer that may be used is a tunnel barrier, configuredfor quantum tunneling magnetoresistance. For example, the tunnel barriermay be composed of an electrically insulating material, such as a metaloxide that may include AlO, TaO, MgO, or any other oxide of aluminum,tantalum, titanium, magnesium, or other appropriate elements orcombinations thereof, or other electrically insulating materials.Another type of interlayer that may be used is an electricallyconducting metal layer configured for giant magnetoresistance. Such anintermediate layer may be composed of a high-conductance metal such asgold, silver, copper, or aluminum, for example. Other types ofintermediate layers configured for other purposes, such as interlayerscomposed of a semiconductor, may be used in various other embodiments.

Various embodiments may also combine quantum tunneling magnetoresistanceand giant magnetoresistance techniques in the same cell. This may becorrelated with optimizing both read and write operations on the samemagnetic cell with the same single terminals 111, 113 on each end of themagnetic cell, in an illustrative embodiment. For example, in anillustrative embodiment corresponding with FIG. 1, first interlayer 131may be composed of an insulating material for quantum tunnelingmagnetoresistance, while second interlayer 132 may be composed of aconducing material for giant magnetoresistance. In this illustrativeembodiment, first interlayer 131, which separates free magnetic layer122 from the fixed magnetic layer 121 that has a parallel easy axis andwhich is used to set the magnetization direction of the free magneticlayer 122, by being composed of an insulating material for quantumtunneling magnetoresistance, may provide for a larger output signal fora read operation. On the other hand, second interlayer 132, whichseparates free magnetic layer 122 from the fixed magnetic layer 123 thathas a perpendicular easy axis and which is used for providing theinitial torque boost on the magnetization direction of the free magneticlayer 122, by being composed of a conducting material for giantmagnetoresistance, could provide for a lower total impedance of themagnetic cell, among other advantages.

The magnetic cell 100 is thereby enabled to provide dual, complementaryspin torques to the free magnetic layer 122, to use a relatively lowelectric current to store data in magnetic cell 100 in the form of whichdirection along the easy axis the magnetization direction of freemagnetic layer 122 is oriented. How magnetic cell 100 provides thesecomplementary torques and thereby encodes data with relatively lowcurrent is further described with reference to FIG. 2.

FIG. 2 depicts a side plan view of magnetic data storage cell 100corresponding to the embodiment depicted in FIG. 1, with schematicdepictions of magnetization directions 141, 142A, 142B, 143, and of spintorques 241, 243 being applied in the cell. In FIG. 2, a spin-polarizedcurrent is being passed through magnetic cell 100, with current runningfrom terminal 113 to terminal 111, i.e. with electrons propagating fromterminal 111 to terminal 113. As this spin-polarized current passes fromfixed magnetic layer 121 with magnetization direction 141, it exerts aspin torque 241 on free magnetic layer 122; and as this spin-polarizedcurrent passes from fixed magnetic layer 123 with magnetizationdirection 143, it exerts a spin torque 243 on free magnetic layer 122.As depicted in FIG. 2, free magnetic layer 122 initially hasmagnetization direction 142A, oriented antiparallel to the magnetizationdirection of fixed magnetic layer 121, i.e. oriented downward asdepicted. The action of the spin-polarized current passing through themagnetic cell 100, and the spin torques 241, 243 resulting from thespin-polarized current associated with the fixed magnetic layers 121,123, causes the magnetization direction of free magnetic layer 122 toflip to magnetization direction 142B, parallel to the magnetizationdirection 141 of fixed magnetic layer 121.

Specifically, the spin torque 243 gives an initial spin torque boost tomagnetization direction 142A to knock it off of the easy axis of freemagnetic layer 122 more rapidly and with more initial torque than wouldbe possible with the spin torque 241 from fixed magnetic layer 121alone; while the spin torque 241 from fixed magnetic layer 121 providesmore torque during the middle of the process of reversing themagnetization direction of free magnetic layer 122, and determines thefinal magnetization direction 142B of free magnetic layer 122 at the endof the write process. These aspects are further explained with referenceto FIG. 3 and FIG. 4, below.

The capability of manipulating the magnetic orientation of the magneticlayers is discussed in additional detail as follows. When aspin-polarized current passes through a magnetic material, the transferof angular momentum from the spins exerts a torque on the magnetizationdirection of the material. In magnetic stacks with fixed magneticlayers, and a free layer, such as fixed layers 121, 123 and free layer122 of FIG. 1 and FIG. 2, the spin-polarized current transfers angularmomentum from the magnetization of each of the fixed layers to the freelayer, exerting a torque on the magnetization of the free layer. In themagnetic element 110 the current is driven vertically through the stack,between terminals 111 and 113, such that for a positive bias (electronflow from lower terminal 111 to upper terminal 113), spin torque drivesthe free layer 122 to a final magnetization direction 142B parallel tothe magnetization of the fixed layer 121 with the parallel easy axis, asdepicted in FIG. 2. For a negative current bias (electron flow fromupper terminal 113 to lower terminal 111), spin torque drives the freelayer 122 to a final magnetization direction 142A antiparallel to themagnetization direction of the fixed layer 121 with the parallel easyaxis (i.e. the opposite of the process depicted in FIG. 2).

The Landau-Lifshitz-Gilbert Equation is applicable to describe thiseffect on the free layer dynamics for the free magnetic layer withreference to each of the fixed magnetic layers, by incorporating theeffects of the magnetization from a spin-polarized current, so the rateof change of the free magnetic layer 122 can be determined as follows:

$\frac{\mathbb{d}{\overset{\rightarrow}{M}}_{free}}{\mathbb{d}t} = {{{- \frac{\mu_{0}\gamma\;\overset{\rightarrow}{M}}{\left( {1 + \alpha^{2}} \right)}} \times \overset{\rightarrow}{H}} - {\frac{\mu_{0}\gamma\;\alpha}{M_{S_{free}}\left( {1 + \alpha^{2}} \right)}{\overset{\rightarrow}{M}}_{free} \times \left( {{\overset{\rightarrow}{M}}_{free} \times \overset{\rightarrow}{H}} \right)} + {\frac{\hslash}{2\; e}\frac{\left( {ɛ\; I} \right)}{V}\frac{\gamma}{M_{S_{free}}^{2}M_{S_{fixed}}}{\overset{\rightarrow}{M}}_{free} \times \left( {{\overset{\rightarrow}{M}}_{free} \times {\overset{\rightarrow}{M}}_{fixed}} \right)}}$where I is the current flowing perpendicular to the plane (CPP) of themagnetic layers, M_(sfree) is the free-layer saturation magnetization,M_(sfixed) is that of the fixed layer, ε is an efficiency factor relatedto the spin polarization of the current, V is the volume of the freelayer, and μ₀ is the magnetic permeability of free space. Solutions tothis equation yield a critical current density, J_(c), beyond which themagnetization of the free layer can be driven either parallel orantiparallel to the fixed layer having the parallel easy axis, dependingon the direction of current flow.

With magnetic cell 100 of FIG. 1 and FIG. 2, each of the two fixedmagnetic layers 121, 123 exerts a torque on the magnetization of freemagnetic layer 122, and both the rate of change in magnetization 142 ofthe free magnetic layer 122 and the critical current density must befigured as determined by the equation above with contributions from eachof the fixed magnetic layers 121, 123, which drives the rate of changeof the magnetization of free magnetic layer 122 much higher, and thecritical density much lower, than with a single fixed magnetic layer.(While the magnetization direction of free magnetic layer 122 isdepicted in particular orientations labeled 142A and 142B in FIG. 2, itis referred to generically herein as magnetization direction 142 ormagnetization 142.) In particular, the torque on the magnetization offree magnetic layer 122 is approximately proportional to the crossproducts of the magnetizations of an adjacent magnetic layer and of thefree magnetic layer, i.e. the magnitudes of the magnetizations times thesine of the angle between them. This is illustrated in graph 300 of FIG.3.

FIG. 3 depicts a graphical representation 300 of a superposition ofmultiple spin torques acting within a cell as a function of anglesbetween magnetization directions, in accordance with an illustrativeexample. In particular, with reference to the illustrative embodiment ofFIGS. 1 and 2, torque component 311 represents the magnitude of the spintorque exerted on the magnetization direction of free magnetic layer 122by fixed magnetic layer 121, and torque component 313 represents themagnitude of the spin torque exerted on the magnetization direction offree magnetic layer 122 by fixed magnetic layer 123.

The rate of change that the magnetization direction of free magneticlayer 122 would have, if affected only by the single fixed magneticlayers individually, would be proportional to the spin torques asdepicted. Because the magnetization direction 141 of fixed magneticlayer 121 is initially antiparallel or parallel to the magnetizationdirection 142 of free magnetic layer 122, its spin-polarized currentinitially has a very small torque on the magnetization direction 142 offree magnetic layer 122, i.e. it is approximately proportional to zero(i.e. the sine of zero), and in actuality it is proportional tosmall-scale corrections to the modeling of the torque that make it notquite zero. The torque and corresponding rate of change of themagnetization direction 142 of free magnetic layer 122 due to theparallel-moment fixed magnetic layer 121 are therefore quite low at theinitiation of a write process. In a magnetic cell with only a singlefixed magnetic layer, this very low initial rate of change would be asubstantial constraint on the speed and performance of the entiremagnetic cell, and of any device that incorporated such magnetic cells.As depicted in component 311 of graph 300, the torque associated withmagnetization direction 141 of fixed magnetic layer 121 then rises untilreaching the full magnitude of the product of the magnetizations 141,142 of fixed magnetic layer 121 and free magnetic layer 122respectively, in the middle of a write process, when the magnetizationdirection of the free magnetic layer 122 is in the middle of flippingand is perpendicular to the magnetization direction 141 of fixedmagnetic layer 121.

At the same time, as depicted in component 313 of graph 300, themagnetization direction 143 of perpendicular-moment fixed magnetic layer123 is initially perpendicular to the magnetization direction 142 offree magnetic layer 122. The spin angular momentum associated withmagnetization direction 143 therefore provides the maximum torque on themagnetization direction 142 of free magnetic layer 122 at the initiationof a write process, i.e. its torque is approximately proportional to 1(i.e. the sine of 90 degrees) times the product of the magnitudes ofmagnetizations 142, 143 of free magnetic layer 122 and fixed magneticlayer 123 respectively, at the initiation of the write process. Thetorque from the perpendicular-moment fixed magnetic layer 123 also risesagain at the end of the write process, when the torque from theparallel-moment fixed magnetic layer 121 is dropping again.

The parallel-moment fixed magnetic layer 121 and theperpendicular-moment fixed magnetic layer 123 therefore exert torquesthat are complementary to each other, and together impose a continuouslyhigh torque on the magnetization 142 of free magnetic layer 122throughout a write process. As depicted in graph 300, torque components311 and 313 superpose to form total torque 321, which remains at orabove the maximum torque provided by either fixed magnetic layer alone,throughout the write process. This provides much faster switching of themagnetization direction 142 of the free magnetic layer 122 than ispossible without the dual, complementary fixed magnetic layers ofmagnetic cell 100. Besides a much lower length of time required forswitching, it also enables a much lower level of critical currentrequired for switching. Because the energy required to impose the freelayer magnetization switching for a write operation is proportional tothe product of the time and the square of the current, and both the timeand the current are lower for dual reference layer magnetic cell 100than with a single reference layer, the energy required for a writeoperation is also much lower for magnetic cell 100 than for a cell withonly one fixed magnetic layer.

FIG. 4 depicts a comparative graphical representation 400 ofmagnetoresistance 403 as a function of current 401 for differentmagnetic cells, in accordance with an illustrative example consistentwith the embodiments discussed above. Graph 400 further illustrates thebenefit of the lower required current as discussed above. Graph 400represents certain relationships between current and magnetoresistanceboth for dual reference layer magnetic cell 100, and for a hypotheticalmagnetic cell with only a single fixed magnetic layer and a single freelayer, for comparison. In graph 400, a magnetic cell with a single freemagnetic layer at a point in time may have either a lowermagnetoresistance 411 or a higher magnetoresistance 413, where the lowervalue 411 corresponds to the magnetization direction of the freemagnetic layer being parallel to the magnetization direction of theparallel-easy-axis fixed magnetic layer (such as fixed magnetic layer121 in magnetic cell 100, or which is the only fixed layer in a cellwith a single fixed layer), and the higher value 413 corresponds to themagnetization direction of the free magnetic layer being antiparallel tothe magnetization direction of the parallel-easy-axis fixed magneticlayer. Current must be applied in the positive-x direction to the valueof the critical current to switch the free layer from antiparallel toparallel and drop from higher magnetoresistance 413 to lowermagnetoresistance 411. On the other hand, current must be applied in thenegative-x direction to the value of the critical current to switch thefree layer from parallel to antiparallel and go from lowermagnetoresistance 411 to higher magnetoresistance 413. The value of thecritical current is different for the two cells, however.

In a magnetic cell with only one fixed magnetic layer, the weakness ofthe initial torque (proportional to the lone torque component 311 inFIG. 3) must be compensated for with a large current to switch themagnetization direction of the free magnetic layer. This relativelylarge critical current for a single-fixed-layer cell is indicated asI_(cs) in FIG. 4, and is at larger values of current in both thepositive-x and negative-x directions in graph 400. Current 420 isdepicted for a single-fixed-layer cell that has been provided thecritical current I_(cs) and is switching from higher to lowermagnetoresistance (or vice-versa on the negative-x side). On the otherhand, in a magnetic cell 100 with dual reference layers, one fixedmagnetic layer 121 with a magnetization direction 141 parallel to theeasy axis of the free layer 122, and another fixed magnetic layer 123with a magnetization direction 143 perpendicular to the easy axis offree layer 122, the critical current for switching the magnetizationdirection of free layer 122 is lower. This relatively small criticalcurrent for a dual-fixed-layer cell is indicated as I_(c) in FIG. 4, andis at smaller values of current in both the positive-x and negative-xdirections in graph 400. Current 421 is depicted for a dual-fixed-layercell that has been provided the critical current I_(c) and is switchingfrom higher to lower magnetoresistance (or vice-versa on the negative-xside).

While the discussion above is provided in the context of performingwrite operations to magnetic cell 100, read operations may also beperformed in a similar manner and through the same terminals 111, 113,but with less than the critical current. A read operation may beperformed on magnetic cell 100 by providing a read query current tomagnetic cell 100 with less than the critical current. This read querycurrent experiences either the higher level of magnetoresistance 413 orthe lower level of magnetoresistance 411 in magnetic cell 100, andreturns as a read response voltage equal to the product of the currentand resistance that carries the information of what magnetic state thefree layer 122 is in at that particular magnetic cell. In more physicaldetail, if the magnetic cell has the magnetization direction of the freemagnetic layer 122 aligned parallel to the magnetization direction ofthe determining reference layer, i.e. the fixed magnetic layer 121 withthe magnetization direction parallel to the easy axis of free magneticlayer 122, and a read current of either polarity is provided through thecell, then it experiences a low magnetoresistance, and the outputvoltage is low. On the other hand, if the magnetic cell has themagnetization direction of the free magnetic layer 122 alignedantiparallel to the magnetization direction of fixed magnetic layer 121,and a read current is provided through the cell then it experiences ahigh magnetoresistance, and the output voltage is high, as sensedthrough the node 115.

FIG. 5 depicts a perspective view of a magnetic data storage cell 500,with exploded views of magnetic layers 121, 522, 123 of the cell withmagnetization directions 141, 542, 143, in accordance with anotherillustrative example that has some similarities with and somedifferences from the illustrative embodiment of magnetic cell 100 inFIGS. 1 and 2. Magnetic cell 500 has many identical components tomagnetic cell 100 of FIGS. 1 and 2, including fixed magnetic layers 121and 123, interlayers 131 and 132, terminals 111 and 113, and node 115.The magnetization of the first fixed magnetic layer 121 is oriented inthe perpendicular orientation, as depicted by magnetization direction141, and the magnetization of the second fixed magnetic layer 123 isoriented in the diametrical orientation, as depicted by magnetizationdirection 143. Magnetic cell 500 also includes free magnetic layer 522,in which the easy axis is oriented in a diametrical orientation, asdepicted with magnetization direction 542. The easy axis of freemagnetic layer 522 is therefore parallel to the magnetization directionof fixed magnetic layer 123.

As with magnetic cell 100, magnetic cell 500 may also exert simultaneousperpendicular spin torques on free magnetic layer 522, i.e. spin torquesthat are perpendicular to each other and where one of them isperpendicular to the easy axis of free magnetic layer 522 and one spintorque is parallel to the easy axis of free layer 522. In this case, itis fixed magnetic layer 123 with the magnetization direction that isparallel to the easy axis of free magnetic layer 522 and that providesspin torque that is parallel or antiparallel to the magnetizationdirection of free layer 522, while fixed magnetic layer 121 has themagnetization direction that is orthogonal to the easy axis of freemagnetic layer 522 and that provides spin torque that is orthogonal tothe magnetization direction of free layer 522. Having the free layer 522with diametrical easy axis may be advantageous in various illustrativeembodiments; for example when the free layer has a relatively high ratiobetween its radius and its thickness, the diametrical orientation may bemore natural and hold its magnetization direction with more stability,in various embodiments.

FIG. 6 depicts a perspective view of a magnetic data storage cell 600,with exploded views of magnetic layers 621, 522, 123 of the cell withmagnetization directions 641, 542, 143, in accordance with anotherillustrative example, in which all three magnetic layers have theirmagnetization directions and easy axes in diametrical or in-planeorientations. Again, many of the components are the same as in magneticcells 100 and 500 of FIGS. 1, 2, and 5, including fixed magnetic layer123 as in magnetic cell 100, and free magnetic layer 522 as in magneticcell 500. Magnetic cell 600 also has fixed magnetic layer 621, in whichthe magnetization is oriented in a second diametrical orientation thatis oriented substantially orthogonal to the diametrical orientation ofthe magnetization of fixed magnetic layer 123 and of the easy axis offree magnetic layer 522. While all the magnetization directions are nowin diametrical orientations, the same pattern applies once again inwhich one of the fixed magnetic layers (123) has its magnetizationdirection parallel to the easy axis of the free magnetic layer (522),and the other fixed magnetic layer (621) has its magnetization directionorthogonal to the easy axis of the free magnetic layer. Therefore, inthe same way once again, spin-polarized currents provided through themagnetic cell 600 may be used to switch the magnetization direction ofthe free magnetic layer 522, where orthogonal-axis fixed magnetic layer621 acts as the torque booster layer, to complement the determiningreference layer to provide the large initial boost of torque to enable afaster switching process with a lower current, while parallel-axis fixedmagnetic layer 123 acts as the determining reference layer, providingthe torque in the orientation in which the magnetization of free layer522 will come to rest. The embodiment of magnetic cell 600 may provideunique advantages in various embodiments, for example when all thelayers have a relatively high ratio of radius to thickness, so that thediametrical orientation may be more natural and may be maintained withhigher stability and lower energy or lesser magnetic pinning resourcesin the case of the fixed magnetic layers 621, 123, as an illustrativeexample.

FIG. 7 depicts a data storage system 700 comprising magnetic datastorage cells, in accordance with another illustrative example. Datastorage system 700 includes a plurality of magnetic data storage cells,such as the illustrative sample 701 of data storage cells 702 shown in amagnified internal view from within data storage system 700. Theillustrative sample 701 of data storage cells 702 is not represented toscale, and various embodiments of data storage systems may include anynumber, potentially up through the millions, billions, trillions, or farmore, of operably connected dual-reference-layer magnetic data storagecells, like any of magnetic cells 100, 500, 600 discussed above. Andwhile data storage system 700 is depicted as a single device in theillustrative example of FIG. 7, other embodiments of data storagesystems may include any number of networked or otherwise connected datastorage devices, and may include a variety of different types of devicesincluding some comprising compound magnetic data storage cells andothers not, distributed over any volume of space. While the array ofdata storage cells 702 depicted in sample 701 with operable signal linesconnected to all the nodes and terminals (like node 115 and terminals111, 113 in the embodiments discussed above) of the cells 702, thisdepiction is simplified, and any operable design for sending signals toand receiving signals from the individual magnetic cells may be employedin different embodiments.

Magnetic data storage cells 702 contained within data storage system 700include representative magnetic cell 702 n, which is depicted in aseparate and further magnified view. Representative magnetic datastorage cell 702 n includes first terminal 711 n, second terminal 713 n,and magnetic cell 702 n that includes three substantially cylindricalmagnetic layers 721 n, 722 n, and 723 n, along with interlayers 731 n,732 n positioned between the adjacent pairs of magnetic layers. Magneticlayers 721 n and 723 n are fixed magnetic layers, while positionedbetween them is free magnetic layer 722 n. Magnetic cell 710 n andmagnetic layers 721 n, 722 n, and 723 n may take the form of any ofmagnetic cells 100, 500, or 600 of the embodiments discussed above, withtheir respective magnetic layers, or any other analogous magnetic celland arrangement of magnetic layers.

In magnetic cell 702 n, terminal 711 n and terminal 713 n are configuredfor providing a spin-polarized current through the magnetic data storagecell 702 n. Fixed magnetic layer 721 n is positioned proximate to thefirst terminal 711 n, and fixed magnetic layer 723 n is positionedproximate to the second terminal 713 n. “Proximate” to a terminal maymean connected to, or at least substantially closer than any of theother layers are, while it may be consistent with additional layers,coatings, sub-terminals, or components within the area around or betweenthe terminals and the magnetic layers, for example. Free magnetic layer722 n is positioned between the first and second fixed magnetic layers721 n, 723 n. As in the embodiments discussed above, one of fixedmagnetic layers 721 n, 723 n has a fixed magnetization direction that issubstantially orthogonal to the magnetization direction of the other oneof fixed magnetic layers 721 n, 723 n, and the free magnetic layer 722 nhas an easy axis that is substantially parallel to a magnetizationdirection of a parallel-axis determining primary reference layer, theprimary reference layer comprising either of fixed magnetic layers 721n, 723 n. This predisposes the free magnetic layer 722 n to have amagnetization direction either parallel or antiparallel to themagnetization direction of the primary reference layer, in accordancewith write operations as discussed above.

The representative magnetic data storage cell 702 n may define avertical axis that runs generally between the terminals 711 n, 713 n andgenerally perpendicular to the magnetic layers 721 n, 722 n, 723 n, inan illustrative embodiment. The magnetization direction of the firstfixed magnetic layer 721 n may be oriented generally parallel to thevertical axis, the magnetization direction of the second fixed magneticlayer 723 n may be oriented either generally parallel or generallyorthogonal to the vertical axis, and the easy axis of the free magneticlayer 722 n may be oriented generally parallel to the magnetizationdirection of either the first fixed magnetic layer or the second fixedmagnetic layer, in analogy to the various embodiments discussed abovewith reference to FIGS. 1, 5, and 6, in various illustrativeembodiments.

Data storage system 700 is configured to provide write signals and readsignals via the signal connections, whereby the write signals causespin-polarized currents having a current density above a critical valueto be provided through the magnetic data storage cells 702 tocontrollably set the magnetization direction of the free magnetic layersof the magnetic data storage cells 702, and the read signals causespin-polarized currents having a current density below the criticalvalue to be provided through the magnetic data storage cells to generatea read output signal that indicates the magnetization direction of thefree magnetic layers 722 of the magnetic data storage cells 702.

In this way, the data storage system 700 may store large amounts of dataencoded in the magnetization directions of the free magnetic layers ofthe many magnetic cells 702 of data storage system 700, and may performread and write operations on the magnetic cells 702 with high speed andlow current, as discussed above. In an illustrative embodiment, thecurrent required may be low enough that it is more compatible withintegrated semiconductor elements that enable operation of the storagecells, such as field effect transistors (FET's) that are typicalcomponents of such a storage device. Additionally, the current requiredmay be low enough that it can be provided by standard integrated circuitcurrent sources, rather than requiring large or bulky devoted currentsources to power the magnetic cells. The low current also contributes tolow rate of energy consumption, low waste heat and a low contribution tosystem cooling requirements, and prolonged battery life in the case ofdata storage in a portable device context. The fast switching times andhigh read and write operations contribute to high-speed performance ofthe data storage system 700.

FIG. 8 depicts a method 800 associated with one of the magnetic cellsdiscussed above, of using a magnetic cell to write and read data, inaccordance with an illustrative example. After starting 801, method 800includes step 803, of providing a free magnetic layer, having first andsecond sides, the free magnetic layer having an initial magnetizationdirection aligned with an easy axis; and step 805, of applying a firstspin torque at the first side to the free magnetic layer and a secondspin torque to the second side of the free magnetic layer, wherein thefirst spin torque is substantially orthogonal to the initialmagnetization direction of the free magnetic layer and the second spintorque is substantially antiparallel to the initial magnetizationdirection of the free magnetic layer. This may serve as a method forwriting data to the magnetic cell. Method 800 may also include a readprocess, including step 807, of applying a read current to the freemagnetic layer; and step 809, of providing an output based at least inpart on a read output signal received in response to the read current.Any kind of data or information may thereby be stored indual-reference-layer magnetic cells, and the data or information may beretrieved by a read process and used to provide a useful or informativeform of tangible output, which may include a display rendered on amonitor, information printed out with a printer, audio data provided toa speaker, a tactile output, or data provided over a hard-wire orwireless signal connection to another computing system, device, router,node, etc. and which may at some point in time be available forincorporation in a user-perceptible output format.

It is to be understood that even though numerous characteristics andadvantages of various aspects of the present disclosure have been setforth in the foregoing description, together with details of thestructure and function of various configurations of the disclosure, thisdisclosure is illustrative only, and changes may be made in details,including in matters of structure and arrangement of parts within theprinciples of the present disclosure to the full extent indicated by thebroad general meaning of the terms in which the appended claims areexpressed.

For example, while magnetic layers and magnetic elements are depicted inthe figures in a cylindrical form and of identical outer radius, variouslayers may be used that are of varying radii relative to each other, andof varying morphologies, particularly of morphologies that may be moreefficient or may be inherent for the underlying crystal lattices of thematerials used, particularly as storage cell size becomes ever smallerin subsequent iterations of development. As another example, while theexamples discussed above make particular mention of cylindrically,one-dimensionally stacked magnetic layers with either perpendicularly ordiametrically oriented magnetizations, other magnetic cells may be usedin which various layers are stacked or arranged adjacent to each otherin any arrangement, in multiple dimensions, and in which layers are usedthat may also be fixed in perpendicularly or diametrically orientedmagnetizations or in an angular or “vortex” orientation of itsmagnetization, in which the magnetization direction curves circularlyaround the central vertical axis of the layer.

Various embodiments may also use layers in which their magnetizationdirections or easy axis are bi-stable in any two orientations; or aretri-stable in all three of the orientations mentioned above, includingout-of-plane and two orthogonal in-plane orientations; or that arebi-stable or otherwise quasi-stable in diametrical orientations alongtwo or more diametrically oriented axes of the layer. Variousembodiments may also use other techniques and structures for definingthe stable or quasi-stable orientations of the magnetization directionor easy axis of any of the magnetic layers, such as inner cavitiesdefining inner annular radii of the layers, for example.

As yet another example, while the embodiments discussed above arediscussed in the context of a magnetic cell with a single magnetic freelayer, other embodiments may include a magnetic cell with multiplediscrete magnetic free layers, in which a write operation can vary thecurrent and duration of time the current is applied to a cell, inaddition to the direction of the current, to controllably switch themagnetic free layers in any combination, thereby writing multiple bitsof information to each single magnetic cell, according to anillustrative embodiment. The free magnetic layers may have interlayersseparating each adjacent pair of free magnetic layers, in anillustrative embodiment, which may define discrete domain walls betweenadjacent free magnetic layers with opposing magnetization directions.Free magnetic layers that have their magnetization directions parallelto the primary, parallel-axis fixed layer may contribute to the spintorque of that parallel-axis fixed layer in switching free layersfurther down the stack of the magnetic cell, in an illustrativeembodiment. Magnetic cells with multiple free magnetic layers mayprovide for relatively larger cells but with more bits stored per cell,and may thereby provide higher overall data storage density, in thisillustrative embodiment.

As yet another example, a data storage cell or data storage system ofthe present disclosure may be used in association with any technologyfor the storage and/or manipulation of data, including those involvingmagnetoresistance, giant magnetoresistance, colossal magnetoresistance,flash memory, optics, magneto-optics, photonics, spintronics,holography, and any other technology. Various embodiments may also beincorporated in multi-technology devices that store or otherwisemanipulate data with different components using magnetic cells as wellas other technologies, such as disc drives or flash drives, for storingor manipulating different portions of data. In addition, the presentdisclosure is not limited to systems for storage or manipulation ofdata, but may also involve any technology involved with spin torquemagnetic manipulation.

1. A magnetic cell comprising: a first fixed magnetic layer; a secondfixed magnetic layer; a free magnetic layer positioned between the firstand second fixed magnetic layers; and terminals configured for providinga spin-polarized current through the magnetic layers wherein themagnetic cell defines a perpendicular orientation that is perpendicularto the layers and a diametrical orientation that is planar with thelayers, and the magnetization direction of the first fixed magneticlayer and the easy axis of the free magnetic layer are oriented in theperpendicular orientation, and the magnetization direction of the secondfixed magnetic layer is oriented in the diametrical orientation.
 2. Themagnetic cell of claim 1, wherein the first fixed magnetic layer has amagnetization direction that is substantially parallel to an easy axisof the free magnetic layer, and the second fixed magnetic layer has amagnetization direction that is substantially orthogonal to the easyaxis of the free magnetic layer.
 3. The magnetic cell of claim 1,wherein at least one of the magnetic layers are composed at least inpart of a ferromagnetic material.
 4. The magnetic cell of claim 3,wherein the ferromagnetic material comprises at least one of iron,cobalt, nickel, boron, lanthanide, neodymium, samarium, a Permalloy, anda Heusler alloy.
 5. The magnetic cell of claim 1, further comprisinginterlayers between the first fixed magnetic layer and the free magneticlayer and between the free magnetic layer and the second fixed magneticlayer.
 6. The magnetic cell of claim 5, wherein the interlayers comprisea tunnel barrier comprising an electrically insulating material.
 7. Themagnetic cell of claim 6, wherein the insulating material comprises anoxide of one or more of aluminum, tantalum, titanium, or magnesium. 8.The magnetic cell of claim 5, wherein the interlayers comprise anelectrically conducting material.
 9. The magnetic cell of claim 8,wherein the electrically conducting material comprises one or more ofcopper, gold, silver, or aluminum.
 10. The magnetic cell of claim 5,wherein the first fixed magnetic layer has a magnetization directionthat is substantially parallel to an easy axis of the free magneticlayer, and the second fixed magnetic layer has a magnetization directionthat is substantially orthogonal to the easy axis of the free magneticlayer, and wherein the interlayer between the first fixed magnetic layerand the free magnetic layer comprises an electrically insulatingmaterial, and the interlayer between the second fixed magnetic layer andthe free magnetic layer comprises an electrically conducting material.11. The magnetic cell of claim 1, wherein the fixed magnetic layers eachhave a substantially greater magnetic volume than the free magneticlayer.
 12. The magnetic cell of claim 1, wherein the fixed magneticlayers are magnetically pinned.
 13. A magnetic cell comprising: a firstfixed magnetic layer; a second fixed magnetic layer; a free magneticlayer positioned between the first and second fixed magnetic layers; andterminals configured for providing a spin-polarized current through themagnetic layers, wherein the magnetic cell defines a perpendicularorientation that is perpendicular to the layers and a diametricalorientation that is planar with the layers, and the magnetizationdirection of the first fixed magnetic layer is oriented in theperpendicular orientation, and the magnetization direction of the secondfixed magnetic layer and the easy axis of the free magnetic layer areoriented in the diametrical orientation.
 14. A magnetic cell comprising:a first fixed magnetic layer; a second fixed magnetic layer; a freemagnetic layer positioned between the first and second fixed magneticlayers; and terminals configured for providing a spin-polarized currentthrough the magnetic layers, wherein the magnetic cell defines a firstdiametrical orientation that is planar with the layers and a seconddiametrical orientation that is planar with the layers and substantiallyorthogonal to the first diametrical orientation, and the magnetizationdirection of the first fixed magnetic layer is oriented in the firstdiametrical orientation, and the magnetization direction of the secondfixed magnetic layer and the easy axis of the free magnetic layer areoriented in the second diametrical orientation.
 15. A data storagesystem comprising: a plurality of magnetic data storage cells; andoperative signal connections with one or more of the magnetic datastorage cells; wherein a representative one of the magnetic data storagecells comprises: a stack of three or more magnetic layers, operablyconnected to a first terminal and a second terminal configured forproviding a spin-polarized current through the magnetic data storagecell, the three or more magnetic layers comprising a first fixedmagnetic layer proximate to the first terminal, a second fixed magneticlayer proximate to the second terminal, and a free magnetic layerpositioned between the first and second fixed magnetic layers; whereinthe first fixed magnetic layer has a fixed magnetization direction, thesecond fixed magnetic layer has a fixed magnetization direction that issubstantially orthogonal to the magnetization direction of the firstfixed magnetic layer, and the free magnetic layer has an easy axis thatis substantially parallel to a magnetization direction of a primaryreference layer, the primary reference layer comprising either the firstfixed magnetic layer or the second fixed magnetic layer, therebypredisposing the free magnetic layer to have a magnetization directioneither parallel or antiparallel to the magnetization direction of theprimary reference layer; wherein the data storage system is configuredto provide write signals and read signals via the signal connections,whereby the write signals cause spin-polarized currents having a currentdensity above a critical value to be provided through the magnetic datastorage cells to controllably set the magnetization direction of thefree magnetic layers of the magnetic data storage cells, and the readsignals cause spin-polarized currents having a current density below thecritical value to be provided through the magnetic data storage cells togenerate a read output signal that indicates the magnetization directionof the free magnetic layers of the magnetic data storage cells.
 16. Thedata storage system of claim 15, wherein the representative magneticdata storage cell defines a vertical axis that runs generally betweenthe terminals and generally perpendicular to the magnetic layers, andthe magnetization direction of the first fixed magnetic layer isoriented generally parallel to the vertical axis, the magnetizationdirection of the second fixed magnetic layer is oriented eithergenerally parallel or generally orthogonal to the vertical axis, and theeasy axis of the free magnetic layer is oriented generally parallel tothe magnetization direction of either the first fixed magnetic layer orthe second fixed magnetic layer.
 17. The data storage system of claim15, wherein the representative magnetic data storage cell defines avertical axis that runs generally between the terminals and generallyperpendicular to the magnetic layers, the magnetization direction of thefirst fixed magnetic layer is oriented generally orthogonal to thevertical axis, the magnetization direction of the second fixed magneticlayer is oriented generally orthogonal to the vertical axis andgenerally orthogonal to the magnetization direction of the first fixedmagnetic layer, and the easy axis of the free magnetic layer is orientedgenerally parallel to the magnetization direction of either the firstfixed magnetic layer or the second fixed magnetic layer.